Tech Guides

Intrinsic Accuracy and Error Maps

As a 3-axis orthogonal moving structure, every CMM has inherent errors that produce measurement uncertainties because no structure is perfect. CMM builders attempt to correct these errors by measuring the inaccuracies and fixing them in software with an “Error Map”. Successful error mapping depends on having a highly repeatable system because an inaccurate structure can be corrected with the error map only if it measures with repeatability. Error mapping is usually efficient to perform at the factory, but it is a more involved and expensive task in the field.

There are (21) degrees of freedom that contribute to the inaccuracy of a CMM structure: for each axis there is linearity, straightness, flatness, roll, pitch, and yaw. The axes also must be square to each other in the XY, YZ and XZ planes. Producing the error map data requires highly accurate measurements of all twenty-one variables in a proper environment using very precise instruments.

It is Helmel’s philosophy to construct our CMMs so that they are physically straight, flat, square, and otherwise true without benefit of an error map. We correct only for linearity in each scale. This is what we mean when we say our CMMs have “Intrinsic Mechanical Accuracy”. When it is necessary or beneficial to a purpose, we can apply an error map, in part or in full, to achieve desired accuracy but this is in very few cases.

Customers benefit from our Intrinsic Mechanical Accuracy in three ways:

• Calibration is simplified because it is reduced to non-linear scale correction in X, Y, and Z plus squaring of the three base planes which can be accomplished with a ball-bar. Typically there are local service sources with competency in these skills. Expensive full remapping is not required.
• We do not secure our linear correction data files so any outside service can perform calibration. Other OEMs do not allow access to calibration files eliminating a customer’s options for calibration and resulting in higher costs.
• Our CMMs can actually be used for mechanical measurements such as straightness, squareness, or flatness with indicators due to the intrinsic accuracy.